In the prior art, for controlling thread tension, a thread tension dial of an upper thread tension device was controlled by trial stitching such that in a straight stitching, an upper thread and a lower thread were crossed at a center in thickness of a fabric as seen in FIG. 6(a), and in a zigzag stitching or one cyclic stitching the upper thread appeared at a rear side of the fabric as seen in FIG. 7(a).
For automatic thread tension control, there have been many proposals of controlling an upper thread tension or an upper thread supply in response to fabric thickness, thread thickness or types of pattern to be stitched
With the above-mentioned control, the crossing points are confirmed by a trial stitching. This is why the upper thread tension control is not sufficient without different properties of the threads being considered as they may cause variations in the thread elongation.
With respect to changing in the thread crossing points, if the stitching is carried out at the same position of the tension dial, i.e., at the same pressure with the threads having a small elongation, the upper thread appears at the rear side of the fabric as seen in FIG. 6(c), i.e., resulting in the lower crossing, but on the other hand, in a case of the thread having a substantial elongation, the upper thread appears at the rear side of the fabric as seen in FIG. 6(b), i.e., resulting in the upper crossing